UPDATE: for some reason, the security level in my R-CLIMIT script was set to 8, so Level 9 was not acceptable. this is now fixed.
For some reason of late a level 9 password will not allow me to override a Credit Term period.
I have to go back into the customer, change the Credit Term, Save the Customer Record, then I can save the Invoice.
I don't like doing this because the incorrect Credit Term is then printed on the invoice, giving the customer some liberties that they are not entitled to.